Spectral sensitization of silver halides with sensitizing dyes is a well-known technique. Generally employed dyes for spectral sensitization include methine dyes, such as cyanine, merocyanine, complex cyanine, and complex merocyanine dyes, etc. These dyes may be used in combination for the purpose of expansion of a color sensitive wavelength region or supersensitization.
Any of these sensitizing dyes is required to have absorbability onto silver halide grain surfaces to function as an electron injection type dye. On the other hand, however, it is known that the sensitizing dyes have certain limits in adsorption to silver halide grain surfaces, and adsorption to saturation or near saturation often results in serious desensitization (inherent desensitization), as described, e.g., in W. C. Lewis et al., Photographic Science and Engineering, Vol. 13, p. 54 (1969). Moreover, surface coating of silver halide grains with sensitizing dyes is sometimes accompanied by problems, such as development inhibition. In accordance with the present invention, therefore, the individual silver halide grains exhibit an extremely low rate of absorption (utilizing efficiency) of incident photons in the spectral sensitization region.
Bird et al. proposed to increase the quantity of absorbed photons by having plural dyes adsorbed on silver halide to form multiple layers as disclosed in U.S. Pat. No. 3,622,316 or by having sensitizing dye molecules containing plural cyanine chromophoric groups adsorbed on silver halide as disclosed in U.S. Pat. Nos. 3,622,317 and 3,976,493, to thereby effect sensitization utilizing Forster type excited energy transfer. However, these techniques still suffer from the aforesaid limitations in adsorption surface area and the disadvantages due to inherent desensitization, and thus attain virtually no substantial positive effects.
Steiger et al. proposed a sensitization technique in which a fluorescent dye, such as a cyanine dye, a xanthene dye, etc., is chemically bonded to colloidal molecules of a dispersion medium, such as gelatin. The fluorescent dye bound to, e.g., gelatin, excites the dye adsorbed on the silver halide surfaces or a spectral sensitizing dye of a different kind through Forster type energy transfer (cf. Th. Forster, Disc. Faraday Soc., Vol. 27, 7 (1959)) or optical absorption of luminescence emitted from the dye bound to gelatin as disclosed in Photo. Sci. Eng., Vol. 27, 59 (1983) and JP-A No. 51-117619 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This technique differs from the system of Bird et al. in that a dye which is not directly adsorbed on silver halide grains also contributes to sensitization. However, since the sensitizing dye to be dispersed in a medium according to the method of Steiger et al. naturally exhibits strong absorbability, a part of the dye bonded to gelatin is also adsorbed directly on the silver halide grains and thereby acts as an energy acceptor. As a result, it is generally difficult to realize ideal overlapping of the luminescence band by the non-adsorbed dye and the absorption band by the adsorbed dye.
This difficulty greatly bars highly efficient energy transmission, because energy transmission essentially requires, in principal, an overlap of a luminescence band and an absorption band, whether it is effected by Forster type energy transfer or reabsorption of luminescence. Further, when the dye to be used is of such a type that adsorption onto the silver halide grains brings about desensitization, the above-described method cannot be applied. Further, this method involves complicated steps, such as syntheses or purification of a dye capable of being bonded to a dispersion medium, entailing greatly increased production cost.
Moreover, in carrying out the above-described method, it is necessary to use a light-collecting dye in high concentration. However, the rate of reaction between the dye and dispersion medium molecules has its limit. Even if a high rate of reaction is achieved, functional groups for hardening would be lost upon reacting, so that a sufficient degree of hardening is hardly obtained. These restrictions impose a practical limit on the concentration of the dye to be added.
Further, the choices available for synthesis of or selection of the aforesaid luminescent dye materials capable of being bonded to a dispersion medium are far narrower in scope than that permitted in the technique of the present invention in which a desired amount of a water-soluble luminescent dye is merely added and dispersed in a hydrophilic medium.
The luminescent dye is required to almost completely decolorize during photographic processing. However, when the dye is chemically bound to the medium as in the above-described method, complete decolorization is virtually impossible or requires a special processing step.
The above-described sensitization method of utilizing dye adsorption in multiple layers and the method of using a dye bound to a binder both lack the ability to increase sensitization efficiency by separating the function of a spectral sensitizing dye (electron injection type) in an adsorbed state from the function of a light-collecting dye of the energy transmission type. These methods are also disadvantageous in that a complicated synthesis route for making the dye is involved, or general development processing is inapplicable.
Under these circumstances, the present inventors have developed a technique which, as shown in JP-A Nos. 63-138341 and 63-138342 comprising clearly separating the light-collecting function of a luminescent dye and the function of sensitization on the surface of silver halide grains, thereby achieving a marked improvement in the efficiency of sensitization by light collection and great facility in the decoloration of a light-collecting dye without requiring any special processing step.
This technique of sensitization by light collection, however, still suffers the problem of "density-dependent light extinction" which occurs when the addition of a light-collecting dye exceeds a certain limit. At a high concentration of light-collecting dye, the energy of its excitation is absorbed by the extinction center and gradual desensitization occurs. Another problem is the desensitization that occurs when part of the light-collecting dye is adsorbed on the surface of silver halide grains.
As described above, the problems of density-dependent light extinction and the desensitization due to adsorption of a light-collecting dye are yet to be solved in connection with prior art techniques for sensitization of silver halide light-sensitive materials by light collection and there has been a strong need to effectively solve these problems.